in- and un-

in- and un-

1. Both prefixes are used to make negative forms of adjectives and nouns; in- is Latin in origin and is no longer active in making new words, whereas un- is English in origin and is a living prefix. Historically, some words have existed in both in- and un- forms, but one or other of them normally drops out in the end, so that (for example) unability gave way to inability in the 18c, although unable and not inable is the corresponding adjective. Other mismatches occur with the pairs imbalance / unbalanced, uncomprehending / incomprehensible, indigestible / undigested, indisputable / undisputed, indistinguishable / undistinguished, unseparated / inseparable, instability / unstable, and others.

2. Choice between un- and in- forms is not normally a problem for native speakers of English, with occasional exceptions such as inadvisable (= not advised) / unadvisable = (of a person) not open to advice, inarguable / unarguable (both in use), incommunicative / uncommunicative (both in use), inconsolable / unconsolable (the first now preferred), indecipherable / undecipherable (both in use), inhuman (= brutal, unfeeling) / unhuman (not human), insupportable (= intolerable) / unsupportable (= indefensible), and others.

3. Note that in- is not used to form negative forms of verbs, whereas un- is (undo, unmask, unsettle, etc.).